Effect of Temperature on Magnetic Properties

Magnetic properties of materials are not constant—they can change significantly with temperature. Every material, whether diamagnetic, paramagnetic, or ferromagnetic, shows a different response to heating or cooling.

For example:

• A ferromagnetic material like iron becomes paramagnetic beyond a certain critical temperature.
• Paramagnetic materials become less magnetic when heated.
• Diamagnetic materials remain largely unaffected by temperature changes.

Understanding how temperature affects magnetism is essential for studying electromagnetism and its practical applications in devices like motors, transformers, magnetic storage systems, and medical technologies.

1.0Basics of Magnetism in Materials

Magnetic behavior in materials arises due to:

• Spin motion of electrons

•  Orbital motion of electrons

•  Alignment of magnetic dipoles

Different materials respond differently:

• Diamagnetic: Weakly repelled by magnetic fields.
• Paramagnetic: Weakly attracted to magnetic fields.
• Ferromagnetic: Strongly attracted and can retain magnetization.

When temperature changes, these atomic and molecular interactions change, altering the net magnetic behavior.

2.0Temperature and Magnetic Behavior

Temperature affects magnetism mainly due to:

• Increased thermal agitation: At higher temperatures, atomic dipoles tend to lose alignment.
• Reduced cooperative effects: In ferromagnetic materials, the collective alignment weakens with heat.
• Critical points: Specific temperatures (like the Curie point) mark drastic changes in magnetic properties.

Thus, the magnetic properties of any material depend on the balance between thermal motion and magnetic interactions.

3.0Effect of Temperature on Diamagnetic Materials

• Diamagnetism arises due to the induced magnetic moments opposing the applied field.
• Since diamagnetism is not based on alignment of dipoles but on electron orbit modifications, temperature has negligible effect.
• Copper, bismuth, silver, and water remain diamagnetic at almost all practical temperatures.                                                   

4.0Effect of Temperature on Paramagnetic Materials 

• Paramagnetism comes from the partial alignment of atomic dipoles in the presence of a magnetic field.
• As temperature increases:
• Thermal agitation increases.
• Dipoles get randomized more easily.
• Net magnetization decreases.
• Hence, paramagnetism decreases with rise in temperature.

This effect is quantitatively explained by Curie’s Law.

5.0Effect of Temperature on Ferromagnetic Materials

Ferromagnetism depends on strong exchange interactions that align magnetic dipoles parallel to each other.

(a) At low to moderate temperature

• Magnetic domains remain strongly aligned.
• Material retains high magnetization.

(b) As temperature rises

• Thermal vibrations disturb the ordered alignment.
• Magnetization gradually decreases.

(c) Curie Temperature

•  At a critical temperature, known as Curie temperature ($T_c$), ferromagnetic behavior vanishes.
• Above this temperature, the material becomes paramagnetic.
• Examples:
• Iron ($T_c$ ≈ 770 °C)
• Nickel ($T_c$ ≈ 358 °C)
• Cobalt ($T_c$ ≈ 1120 °C)

(d) Beyond Curie Point

• Magnetic domains disappear completely.
• Only weak paramagnetic properties remain.

6.0Magnetic Susceptibility and Temperature Dependence

Magnetic susceptibility ($\chi$) indicates how much a material is magnetized in response to an external magnetic field.

• Diamagnetic materials: ( $\chi$ ) is negative and nearly constant with temperature.
• Paramagnetic materials: ( $\chi$ ) decreases with increase in temperature.
• Ferromagnetic materials: ( $\chi$ ) is very high at low temperature, but drops sharply after Curie point.

7.0Curie Law of Paramagnetism

For paramagnetic substances:
$\chi =\frac{C}{T}$

Where:

• ( $\chi$ ) = magnetic susceptibility
• ( C ) = Curie constant (material specific)
• ( T ) = absolute temperature (in Kelvin)

Implication:

• As ( T ) increases, ( $\chi$ ) decreases.
• Hence, paramagnetic materials are more magnetic at lower temperatures.

8.0Curie-Weiss Law of Ferromagnetism

For ferromagnetic substances above Curie temperature:

$\chi =\frac{C}{T-\theta }$

Where:

• ( $\chi$ ) = susceptibility
• ( C ) = Curie constant
• ( $\theta$ ) = Curie temperature

Implication:

• As ( T ) approaches ($\theta$), susceptibility tends to infinity (indicating loss of ferromagnetism).
• Beyond this, the material behaves like a paramagnet.

9.0Applications of Temperature Effects on Magnetism

• Magnetic storage: Hard drives and tapes rely on ferromagnetic materials with stable magnetism at room temperature.
• Transformers and motors: Use soft iron cores, which perform efficiently at lower temperatures.
• Magnetic levitation: Requires precise control of magnetic properties at specific temperatures.
• Superconductors: Show complete expulsion of magnetic fields (Meissner effect) at very low temperatures.
• Magnetic sensors: Calibrated considering temperature variation to avoid measurement errors.